Non-aqueous electrolyte secondary battery and negative electrode for the same

ABSTRACT

A non-aqueous electrolyte battery is provided, which exhibits good high-rate discharge characteristics and low-temperature characteristics and ensures high safety when the negative electrode contains 0.6 to 1.7 parts by weight of a particulate modified styrene-butadiene rubber as a binder and 0.7 to 1.2 parts by weight of a thickening agent so that the total amount of the binder and thickening agent is 1.3 to 2.4 parts by weight per 100 parts by weight of a carbon material as an active material, and the concentration of LiPF 6  in the non-aqueous electrolyte is 0.6 to 1.05 mole/liter. The surface area of the active material effectively contributable to charging and discharging reaction is sufficient when the surface area of the carbon material per 1 g of the binder contained in the negative electrode is 300 to 600 m 2 .

This application is a divisional of application Ser. No. 10/883,727filed Jul. 6, 2004 which is a divisional of application Ser. No.09/845,265 filed May 1, 2001 now U.S. Pat. No. 6,773,838.

BACKGROUND OF THE INVENTION

Non-aqueous electrolyte secondary batteries used as electric powersources for portable electronic equipments in recent years include alithium-containing transition metal oxide in the positive electrode anda carbon material capable of absorbing and desorbing lithium in thenegative electrode, thereby having high power and high energy density.The positive and negative electrodes include respective binders forbinding active material particles together. As the binder in thenegative electrode, used are polyvinylidene difluoride (PVDF) orstyrene-butadiene rubber (SBR), for example.

To impart sufficient strength to the negative electrode, it is necessaryto mix a large amount of binder with a negative electrode activematerial such as the above carbon material. If a large amount of binderis used, however, the surface of the carbon material is covered with thebinder. This reduces the surface area of the carbon materialcontributable to charging and discharging reaction, and thusdeteriorates the high-rate discharge characteristics and thelow-temperature characteristics of the battery. To compensate this, thesalt concentration in the non-aqueous electrolyte must be increased.

However, increase of the salt concentration will enhance the reactivityof the electrolyte when the battery is under a high temperature andovercharged. Therefore, the battery temperature tends to easily rise,and thus the safety may be impaired.

In addition, if most of the surface of the carbon material is so coveredwith the binder that the surface area of the carbon materialcontributable to the charging and discharging reaction reduces, thecarbon material fails to absorb a sufficient amount of Li. As a result,metallic Li is deposited on the surface of the carbon material, and thusthe safety of the battery may further be impaired.

Also, the high-rate discharge characteristics of the batteries aregreatly influenced by the affinities between the non-aqueous electrolyteand the electrodes, which depend on the amount and kind of the binders.If the permeability of the non-aqueous electrolyte into one of theelectrodes is too high, the distribution of the non-aqueous electrolyteinside the battery is nonuniform, and thus the high-rate dischargecharacteristics are impaired.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery. The present invention also relates to a negative electrode fora non-aqueous electrolyte secondary battery.

More particularly, the present invention relates to a non-aqueouselectrode secondary battery having a negative electrode including aspecific binder and a non-aqueous electrolyte with a low saltconcentration, which exhibits good high-rate discharge characteristicsand low-temperature characteristics, and which also ensures high safety.

The present invention also relates to a negative electrode including aspecific binder and an active material, of which the surface areacontributable to the charging and discharging reaction is sufficientwhile securing the strength of the electrode.

Specifically, the present invention relates to a non-aqueous electrolytesecondary battery comprising: a positive electrode comprising a compoundoxide containing lithium; a negative electrode comprising a carbonmaterial; a separator interposed between the positive electrode and thenegative electrode; and a non-aqueous electrolyte comprising anon-aqueous solvent and LiPF₆ dissolved therein, wherein the negativeelectrode contains 0.6 to 1.7 parts by weight of a particulate modifiedstyrene-butadiene rubber and 0.7 to 1.2 parts by weight of a thickeningagent per 100 parts by weight of the carbon material where the totalamount of the particulate modified styrene-butadiene rubber and thethickening agent is 1.3 to 2.4 parts by weight per 100 parts by weightof the carbon material, and the concentration of LiPF₆ in thenon-aqueous electrolyte is 0.6 to 1.05 mole/liter.

Herein, the particulate modified styrene-butadiene rubber preferablycontains a copolymer comprising an acrylonitrile unit, a styrene unit,and a butadiene unit.

All or a part of the copolymer is preferably in a form of a core-shelltype particle.

In a FT-IR absorption spectrum of the copolymer comprising anacrylonitrile unit, a styrene unit, and a butadiene unit, the intensityof the absorption peak attributed to C≡N stretching vibration in theacrylonitrile unit is preferably 0.1 to 2, and more preferably 0.1 to0.5 times the intensity of the absorption peak attributed to C═Cstretching vibration in the butadiene unit.

It is preferable that the mean particle size of the particulate modifiedstyrene-butadiene rubber is 0.05 to 0.4 μm.

It is also preferable that the thickening agent is carboxymethylcellulose.

It is still also preferable that the concentration of LiPF₆ in thenon-aqueous electrolyte is 0.7 to 0.9 mole/liter.

The positive electrode preferably contains 0.4 to 2 parts by weight of aparticulate modified acrylic rubber per 100 parts by weight of thecompound oxide containing lithium.

The particulate modified acrylic rubber preferably contains a copolymercomprising a 2-ethylhexylacrylate unit, an acrylic acid unit, and anacrylonitrile unit.

In a FT-IR absorption spectrum of the copolymer comprising a2-ethylhexylacrylate unit, an acrylic acid unit, and an acrylonitrileunit, the intensity of the absorption peak attributed to C═O stretchingvibration in the 2-ethylhexylacrylate unit and acrylic acid unit ispreferably 3 to 50 times the intensity of the absorption peak attributedto C≡N stretching vibration in the acrylonitrile unit.

The present invention also relates to a negative electrode for anon-aqueous electrolyte secondary battery comprising: a carbon materialas an active material; 0.6 to 1.7 parts by weight of the particulatemodified styrene-butadiene rubber as a binder per 100 parts by weight ofthe carbon material; and 0.7 to 1.2 parts by weight of a thickeningagent per 100 parts by weight of the carbon material, wherein the totalamount of the particulate modified styrene-butadiene rubber and thethickening agent is 1.3 to 2.4 parts by weight per 100 parts by weightof the carbon material.

The present invention still also relates to a negative electrode for anon-aqueous electrolyte secondary battery comprising: a carbon materialas an active material; and the particulate modified styrene-butadienerubber as a binder; wherein the surface area of the carbon material is300 to 600 m² per 1 gram of the particulate modified styrene-butadienerubber.

Note that in the FT-IR absorption spectrum, the intensity of theabsorption peak is obtained as the height of the absorption peak fromthe base line of the spectrum.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view of a rectangular battery as an exampleof the non-aqueous electrolyte secondary battery of the presentinvention.

FIG. 2 is an example of an absorption spectrum obtained by FT-IRmeasurement of a particulate modified styrene-butadiene rubber.

FIG. 3 is an example of a transmission spectrum obtained by FT-IRmeasurement of SBR.

FIG. 4 is an example of an absorption spectrum obtained by FT-IRmeasurement of a particulate modified acrylic rubber.

FIG. 5 is an example of a transmission spectrum obtained by FT-IRmeasurement of a copolymer comprising an ethylene unit and a vinylalcohol unit.

DETAILED DESCRIPTION OF THE INVENTION

The non-aqueous electrolyte secondary battery of the present inventionuses a negative electrode including a specific binder and a thickeningagent at a specific ratio, and a non-aqueous electrolyte with a low saltconcentration, to improve the high-rate discharge characteristics, thelow-temperature characteristics, and the safety of the battery. When apositive electrode including a specific binder of a specific amount isused together with the negative electrode and the non-aqueouselectrolyte described above, a further improved battery is provided.

The negative electrode of the present invention includes a negativeelectrode material mixture and a core member, for example.

The negative electrode material mixture is prepared by blending a carbonmaterial as an active material, a particulate modified styrene-butadienerubber and a thickening agent at a predetermined ratio. The term“modified” of the particulate modified styrene-butadiene rubber usedherein means that the particulate modified styrene-butadiene rubbercontains at least an acrylonitrile unit.

The negative electrode is produced by providing the negative electrodematerial mixture to the core member, which is made of metal foil such ascopper foil or punched metal, rolling the resultant member, and cuttingthe rolled member.

From the viewpoint of reduction in size and weight of the battery, thethickness of the core member is generally about 8 to 20 μm, and thethickness of the negative electrode is generally 80 to 200 μm.

A carbon powder such as a graphite powder is used as the carbon materialthat is the negative electrode active material. In particular, flakegraphite and a spherical artificial graphite are preferably used. Themean particle size of the carbon powder is 20 to 30 μm, for example. Thespecific surface area of the carbon powder is 2 to 5 m²/g, for example.

The particulate modified styrene-butadiene rubber preferably contains acopolymer comprising an acrylonitrile unit, a styrene unit, and abutadiene unit.

The copolymer is preferably in a form of a core-shell type particle. Thecore portion of the core-shell type particle has rubber-like elasticityand is made of a copolymer comprising an acrylonitrile unit, a styreneunit, a butadiene unit and an acrylate unit, for example, which issufficiently cross-linked by use of an appropriate cross linking agent.The shell portion is made of a highly viscous polymer such as acopolymer comprising an acrylate unit and a styrene unit, for example.

The core-shell type particles are obtained in the following two-stageprocess, for example. First, a raw material monomer mixture for the coreportion including a cross linking agent is polymerized to produce alatex. In this process, high modulus of elasticity is imparted to thecore portion by mixing acrylonitrile in the raw material monomer mixturefor the core portion. Thereafter, a raw material monomer mixture for theshell portion is mixed with the latex to conduct graft-copolymerizationto obtain the core-shell type particles.

The above copolymer in a form of a core-shell type particle preferablycomprises an acrylonitrile unit and a butadiene unit so that, in anabsorption spectrum obtained by FT-IR measurement of the copolymer, theintensity of the absorption peak attributed to the C≡N stretchingvibration in the acrylonitrile unit is 0.1 to 2 times the intensity ofthe absorption peak attributed to the C═C stretching vibration in thebutadiene unit. If the intensity of the peak attributed to the C≡Nstretching vibration is less than 0.1 times the intensity of the peakattributed to the C═C stretching vibration, the rubber-like elasticityof the modified styrene-butadiene rubber is insufficient. As a result,the particulate modified styrene-butadiene rubber fails to provide asufficient strength to the negative electrode. In addition, the surfaceof the active material is excessively covered with the modifiedstyrene-butadiene rubber. On the contrary, if the intensity of theabsorption peak attributed to the C≡N stretching vibration is more thantwice the intensity of the absorption peak attributed to the C═Cstretching vibration, the stickiness of the particulate modifiedstyrene-butadiene rubber is insufficient. As a result, the negativeelectrode material mixture tends to easily come off from the coremember.

The mean particle size of the particulate modified styrene-butadienerubber is preferably 0.05 to 0.4 μm. When the mean particle size is inthis range, a sufficiently strong negative electrode can be obtainedusing a small amount of the particulate modified styrene-butadienerubber. If the mean particle size is too small, most of the surface ofthe active material is covered with the particulate modifiedstyrene-butadiene rubber. If the mean particle size is too large, on theother hand, the distance between the adjacent active material particlesis so large that the conductivity inside the negative electrodedecreases.

The amount of the particulate modified styrene-butadiene rubber in thenegative electrode material mixture is 0.6 to 1.7 parts by weight per100 parts by weight of the carbon material as the negative electrodeactive material. If the amount of the particulate modifiedstyrene-butadiene rubber is too small, the negative electrode fails tohave a sufficient strength, and thus the mixture tends to easily comeoff from the core member. If the amount is too large, on the other hand,the reaction surface area of the active material become so small thatthe high-rate discharge characteristics deteriorate.

In the conventional case of using PVDF as the binder, the preferredamount of the binder in the negative electrode material mixture is 5 to10 parts by weight per 100 parts by weight of the carbon material. Inthe case of SBR, the amount is preferably 2 to 5 parts by weight. Thisindicates that the negative electrode material mixture according to thepresent invention includes a significantly-reduced amount of the bindercompared with the conventional negative electrode material mixture.

The surface area of the carbon material included in the negativeelectrode is preferably 300 to 600 m² per 1 gram of the particulatemodified styrene-butadiene rubber included in the negative electrode. Ifthe surface area of the carbon material per 1 gram of the particulatemodified styrene-butadiene rubber is less than 300 m², the activematerial is excessively covered with the binder, resulting indeterioration of the battery charging characteristics and thusshortening the cycle life of the battery. If the surface area is morethan 600 m²/g, the adhesion of the negative electrode material mixtureto the core member decreases due to an insufficient amount of thebinder.

As the thickening agent mixed in the negative electrode materialmixture, used are cellulose type thickening agents such as carboxymethylcellulose (CMC) and/or a copolymer comprising an ethylene unit and avinyl alcohol unit, for example. These may be used alone or incombination of two or more of them. Among these thickening agents, CMCis preferably used.

The amount of the thickening agent in the negative electrode materialmixture is suitably 0.7 to 1.2 parts by weight per 100 parts by weightof the carbon material as the negative electrode active material. If theamount of the thickening agent is too small, the negative electrodematerial mixture fails to become pasty and thus tends to easily come offfrom the core member. If the amount is too large, on the other hand, theactive material is covered with the thickening agent too much, resultingin decrease of the reaction surface area.

Herein, it is effective that the total amount of the particulatemodified styrene-butadiene rubber and the thickening agent should be 1.3to 2.4 parts by weight per 100 parts by weight of the carbon material asthe negative electrode active material. If the total amount is less than1.3 parts by weight, the active material particles fail to besufficiently bound with each other, resulting in insufficient strengthof the negative electrode. If the total amount is too large, the activematerial is excessively covered with the binder and the thickeningagent, resulting in decrease of the reaction surface area.

In the present invention, the positive electrode includes a positiveelectrode material mixture and a core member, for example.

The positive electrode material mixture is prepared by blending apositive electrode active material, a conductive agent and a binder at apredetermined ratio.

The positive electrode is obtained by providing the positive electrodematerial mixture to the core member, which is made of metal foil such asaluminum foil or punched metal, rolling the resultant member, andcutting the rolled member. From the viewpoint of reduction in size andweight of the battery, the thickness of the core member is generallyabout 8 to 20 μm, and the thickness of the positive electrode isgenerally 80 to 200 μm.

The positive electrode active material includes a compound oxidecontaining lithium such as LiCoO₂, LiNiO₂ or LiMn₂O₄, for example. Thesemay be used alone or in combination of two or more of them.

As the conductive agent mixed in the positive electrode materialmixture, used are a natural graphite such as flake graphite, anartificial graphite such as vapor-phase growth graphite and a carbonblack such as acetylene black. These may be used alone or in combinationof two or more of them.

As the binder mixed in the positive electrode material mixture, used area particulate modified acrylic rubber and/or PVDF, for example. Amongthese, a particulate modified acrylic rubber is preferable. Aparticulate modified acrylic rubber is available, for example, in a formof a dispersion with water or an organic solvent as the dispersionmedium. A dispersion using an organic solvent is preferable. The meanparticle size of the particulate modified acrylic rubber is preferably0.05 to 0.3 μm. When the mean particle size is in this range, a positiveelectrode which is nicely balanced in strength, active material densityand porosity is obtained.

The term “modified” of the particulate modified acrylic rubber usedherein means that the particulate modified acrylic rubber contains atleast an acrylonitrile unit.

The particulate modified acrylic rubber contains preferably a copolymercomprising a 2-ethylhexylacrylate unit, an acrylic acid unit, and anacrylonitrile unit. Also preferably, in an absorption spectrum obtainedby FT-IR measurement of the copolymer, the intensity of the absorptionpeak attributed to the C═O stretching vibration in the2-ethylhexylacrylate unit and the acrylic acid unit is 3 to 50 times theintensity of the absorption peak attributed to the C≡N stretchingvibration in the acrylonitrile unit. If the intensity of the peakattributed to the C═O stretching vibration is less than 3 times theintensity of the peak attributed to the C≡N stretching vibration, thestickiness of the particulate modified acrylic rubber is insufficient.If the intensity of the peak attributed to the C═O stretching vibrationis more than 50 times the intensity of the peak attributed to the C≡Nstretching vibration, the rubber-like elasticity of the particulatemodified acrylic rubber is insufficient, and thus the strength of thepositive electrode is lowered.

The particulate modified acrylic rubber is preferably in a form of acore-shell type particle. The core portion of the core-shell typeparticle has rubber-like elasticity, and is made of a copolymercomprising an acrylonitrile unit, for example, which is sufficientlycross-linked by use of an appropriate cross linking agent. The shellportion is made of a highly viscous polymer such as a copolymercomprising a 2-ethylhexylacrylate unit and an acrylic acid unit, forexample. The core-shell type particles can be produced by the similartwo-stage process to that described above.

The amount of the particulate modified acrylic rubber in the positiveelectrode material mixture is preferably 0.4 to 2 parts by weight per100 parts by weight of the positive electrode active material. If theamount of the particulate modified acrylic rubber is too small, thepositive electrode fails to have a sufficient strength, and thus themixture tends to easily come off from the core member. If the amount istoo large, on the other hand, the porosity of the positive electrode islow, and thus the reaction surface area of the active material is small.This deteriorates the high-rate discharge characteristics.

In the non-aqueous electrolyte secondary battery of the presentinvention, by using the particulate modified acrylic rubber as thebinder in the positive electrode material mixture, suitable permeabilityof the non-aqueous electrolyte into both of the negative electrode andthe positive electrode is attained. In addition, the permeability of thepositive electrode side and that of the negative electrode side are wellbalanced, and thus the distribution of the non-aqueous electrolyteinside the battery is uniform. Therefore, the resultant battery exhibitsexcellent low-temperature characteristics and high-rate dischargecharacteristics.

The permeability of the non-aqueous electrolyte into an electrode can beevaluated by observing the contact angle between the surface of theelectrode and the non-aqueous electrolyte. The value of the contactangle is preferably 10 to 30° although the value varies depending on thekind of the non-aqueous electrolyte and active material density of theelectrode. If the contact angle is too small, the electrode absorbs thenon-aqueous electrolyte so excessively that the high-rate dischargecharacteristics of the resultant battery become insufficient. If thecontact angle is too large, on the other hand, the electrode hardlyabsorbs the non-aqueous electrolyte. In this case, also, the high-ratedischarge characteristics of the battery deteriorate.

In the FT-IR measurement, the absorption spectra of the particulatemodified styrene-butadiene rubber and the particulate modified acrylicrubber may be measured by using these materials applied on a KBr plate,respectively. In general, the absorption peak attributed to the C═Cstretching vibration in the butadiene unit is observed near 880 to 940cm⁻¹. The absorption peak attributed to the C═O stretching vibration inthe 2-ethylhexylacrylate unit and the acrylic acid unit is observed near1700 to 1760 cm⁻¹. The absorption peak attributed to the C≡N stretchingvibration in the acrylonitrile unit is observed near 2200 to 2280 cm⁻¹.

The positive electrode and the negative electrode are laminated with aseparator interposed therebetween, to form an electrode group. Theelectrode group may be wound. As the separator, used is a polyethylenemicro-porous film, for example, having a thickness of 10 to 40 μm, ingeneral. In the case of producing a rectangular battery, the woundelectrode group is compressed in diametrical direction to obtain aroughly elliptic section.

FIG. 1 is a transverse cross-sectional view of a rectangular battery asan example of the non-aqueous electrolyte battery of the presentinvention, cut along the plane parallel to the winding direction of theelectrode group. Referring to FIG. 1, a rectangular battery case 1 ischarged with the electrode group. In the electrode group, a sheet-likepositive electrode plate 2 and a sheet-like negative electrode plate 3are laminated with a separator 4 interposed therebetween, wound andcompressed to have a certain ellipticity.

As the non-aqueous solvent used for the non-aqueous electrolyte, thoseconventionally used for lithium ion secondary batteries can be usedwithout any limitation. Examples of such non-aqueous solvents includeethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethylcarbonate, and propylene carbonate. These may be used alone or incombination of two or more of them.

The concentration of LiPF₆ in the non-aqueous electrolyte is 0.6 to 1.05mole/liter. If the concentration of LiPF₆ is less than 0.6 mole/liter,the battery performance is deteriorated. If the concentration is morethan 1.05 mole/liter, the safety of the battery is impaired. In order toobtain a non-aqueous electrolyte secondary battery exhibiting goodhigh-rate discharge characteristics and low-temperature characteristicsand ensuring high safety, the concentration of LiPF₆ is preferably 0.7to 0.9 mole/liter.

Hereinafter, the present invention will be described specifically by wayof examples. It should be noted that the present invention is notlimited to the following examples.

EXAMPLES 1 TO 9 AND COMPARATIVE EXAMPLES 1 TO 10

Batteries A1 to S1 were produced in the manner described below, and thecharacteristics of the resultant batteries were evaluated.

(i) Production of Negative Electrode

Negative electrode material mixtures having predetermined compositionsfor batteries A1 to S1 were prepared using predetermined binders. Flakegraphite was used as the negative electrode active material, andcarboxymethyl cellulose (CMC) was used as the thickening agent. As thebinder, those shown in Table 1 were used. In Table 1, also shown are therespective amounts of the binder and the thickening agent, as well asthe total amount thereof, per 100 parts by weight of the active materialin the negative electrode material mixture for each battery.

TABLE 1 Amount (part(s) by weight) Salt Thickening concentration inExample No. Battery Binder Binder agent Total electrolyte (M)Comparative A1 BM400B 1.2 1.2 2.4 1.1 Ex. 1 Ex. 1 B1 BM400B 1.2 1.2 2.41.05 Ex. 2 C1 BM400B 1 1 2 0.9 Comparative D1 MPE 1 1 2 0.9 Ex. 2Comparative E1 SBR 1 1 2 0.9 Ex. 3 Comparative F1 PVDF 4 0 4 0.9 Ex. 4Ex. 3 G1 BM400B 1 1 2 0.7 Comparative H1 BM400B 0.5 1.2 1.7 0.6 Ex. 5Ex. 4 I1 BM400B 0.6 0.7 1.3 0.6 Ex. 5 J1 BM400B 0.6 1.2 1.8 0.6Comparative K1 BM400B 0.6 1.3 1.9 0.6 Ex. 6 Ex. 6 L1 BM400B 1 1 2 0.6Ex. 7 M1 BM400B 1.2 1.2 2.4 0.6 Ex. 8 N1 BM400B 1.4 1 2.4 0.6Comparative O1 BM400B 1.4 1.2 2.6 0.6 Ex. 7 Ex. 9 P1 BM400B 1.7 0.7 2.40.6 Comparative Q1 BM400B 1.7 0.6 2.3 0.6 Ex. 8 Comparative R1 BM400B1.8 0.7 2.5 0.6 Ex. 9 Comparative S1 BM400B 0.6 0.7 1.3 0.55 Ex. 10 Ex.10 T1 BM400B 1 1 2 0.9

Details of the binders shown in Table 1 are as follows.

BM400B: Particulate modified styrene-butadiene rubber having a meanparticle size of 0.2 μm, manufactured by Nippon Zeon Co., Ltd.

MPE: Modified polyethylene

SBR: Styrene-butadiene rubber (unmodified)

PVDF: Polyvinylidene difluoride

In the absorption spectrum obtained by FT-IR measurement of BM400B, theintensity of the absorption peak attributed to the C≡N stretchingvibration in the acrylonitrile unit is 0.5 times the intensity of theabsorption peak attributed to the C═C stretching vibration in thebutadiene unit.

The absorption spectrum is shown in FIG. 2, which is obtained by themeasurement using microscopic FT-IR; Continu μm with AVATAR-360 as alight source manufactured by Nicolet Instrument Corp., under thefollowing conditions:

Number of sample scans: 32

Number of background scans: 32

Resolving power: 4000

Sample gain: 1.0

A sample for the measurement was prepared by dissolving BM400B inN-methyl-2-pyrrolidone, applying the obtained solution to a KBr plate,and drying the plate.

In FIG. 2, the absorption peak observed near 2237 cm⁻¹ is attributed tothe C≡N stretching vibration in the acrylonitrile unit, and theabsorption peak observed near 911 cm⁻¹ is attributed to the C═Cstretching vibration in the butadiene unit.

For comparison, a transmission spectrum obtained by FT-IR measurement ofcommon unmodified SBR is shown in FIG. 3. The conditions, instrument,and the like are the same as those used for the measurement of thespectrum in FIG. 2. In FIG. 3, there is observed no absorption peakattributed to the C≡N stretching vibration in the acrylonitrile unitnear 2237 cm⁻¹.

Each of the resultant negative electrode material mixture comprising theactive material, the binder, and the thickening agent was applied toboth surfaces of a core member made of 15 μm thick copper foil, rolledto a thickness of 140 μm, and cut to a predetermined length, to obtain anegative electrode. A lead made of the same material as that of the coremember was connected to the negative electrode.

(ii) Production of Positive Electrode

Four parts by weight of PVDF as the binder and 3 parts by weight ofacetylene black as the conductive agent were blended with 100 parts byweight of LiCoO₂, to obtain a positive electrode material mixture. Theresultant positive electrode material mixture was applied to bothsurfaces of a core member made of 20 μm thick aluminum foil, rolled to apredetermined thickness, and cut to a predetermined length, to obtain apositive electrode. A lead made of the same material as that of the coremember was connected to the positive electrode.

(iii) Production of Battery

The positive electrode and each of the negative electrodes obtained inthe above manner were laminated with a separator interposed therebetweenand then wound to obtain an electrode group. A polyethylene micro-porousfilm having a thickness of 27 μm was used as the separator. The woundelectrode group was compressed in diametrical direction to have aroughly elliptic section.

For preparation of a non-aqueous electrolyte, LiPF₆ was dissolved in amixture of equal volumes of ethylene carbonate and ethylmethyl carbonateas a non-aqueous solvent so that the salt concentration by mole/liter(M) was as shown in Table 1 for each battery.

The electrode group was housed in a predetermined aluminum case withinsulating rings placed on the top and bottom surfaces of the electrodegroup with 3.2 g of the non-aqueous electrolyte. The leads of thenegative and positive electrodes were connected to predeterminedpositions, respectively. Then, the opening of the case was sealed with asealing plate, to complete each of the batteries A1 to S1. Each of thebatteries is in the shape of a rectangle having a width of 30 mm, aheight of 48 mm, and a thickness of 5 mm, and has a nominal capacity of600 mAh.

The resultant batteries were evaluated in the following points.

(i) Low-temperature Characteristics

Batteries A1 to S1 were charged until the battery voltage reached 4.2 Vat 600 mA in an atmosphere of 0° C., and the charged capacity (C_(LT))in this state was measured. The results are shown in Table 2.

(ii) High-rate Discharge Characteristics

Batteries A1 to S1 were charged until the battery voltage reached 4.2 Vat 600 mA and then discharged until the battery voltage decreased to 3 Vat 120 mA, in an atmosphere of 20° C. Subsequently, the batteries werecharged until the battery voltage reached 4.2 V at 600 mA and thendischarged until the battery voltage decreased to 3 V at 1200 mA. Thedischarge capacities were measured at the two discharging operations,and the ratio (C₁₂₀₀/C₁₂₀) of the latter to the former was calculated.The results expressed as a percentage are shown in Table 2.

(iii) Capacity Maintenance Rate

For each of the batteries A1 to S1, the operation of charging thebattery until the battery voltage reached 4.2 V at 600 mA and thendischarging the battery until the battery voltage decreased to 3 V at600 mA was repeated 200 times in an atmosphere of 20° C. The ratio(C_(200th)/C_(1st)) of the discharge capacity at the 200th operation tothat at the first operation was calculated. The results expressed as apercentage are shown in Table 2.

(iv) Overcharge Test

For each of the batteries A1 to S1, charging was continued at 1260 mA inan atmosphere of 20° C. and stopped when the battery surface temperaturereached 80° C. The batteries were then left to stand for a while, toexamine the surface temperature. The battery of which the surfacetemperature rose to 90° C. or more was evaluated as “X”, while thebattery of which the surface temperature was less than 90° C. wasevaluated as “◯”. The results are shown in Table 2. The batteriesevaluated as “◯” can be considered to have sufficient safety.

TABLE 2 C_(LT) C₁₂₀₀/C₁₂₀ C_(200th)/C_(1st) Over charge Example No.Battery (mAh) (%) (%) test Comparative A1 297 95 94 X Ex. 1 Ex. 1 B1 29795 93 ◯ Ex. 2 C1 298 95 93 ◯ Comparative D1 208 91 85 X Ex. 2Comparative E1 285 94 92 X Ex. 3 Comparative F1 216 92 87 X Ex. 4 Ex. 3G1 272 86 87 ◯ Comparative H1 — — — — Ex. 5 Ex. 4 I1 249 75 82 ◯ Ex. 5J1 242 73 80 ◯ Comparative K1 146 53 53 X Ex. 6 Ex. 6 L1 235 72 79 ◯ Ex.7 M1 227 71 76 ◯ Ex. 8 N1 200 67 72 ◯ Comparative O1 120 47 45 X Ex. 7Ex. 9 P1 214 70 74 ◯ Comparative Q1 — — — — Ex. 8 Comparative R1 164 6062 X Ex. 9 Comparative S1 148 49 63 ◯ Ex. 10 Ex. 10 T1 320 97 95 ◯

From the results in Table 2, the followings are found.

Among the batteries in which the salt concentration in the non-aqueouselectrolyte is in the range of 0.6 to 1.05 mole/liter, those that usethe particulate modified styrene-butadiene rubber in the negativeelectrode material mixture exhibit high safety. On the contrary, batteryA1 of comparative example 1, in which the salt concentration in thenon-aqueous electrolyte is 1.1 mole/liter, is insufficient in safety.Battery S1 of comparative example 10, in which the salt concentration inthe non-aqueous electrolyte is 0.55 mole/liter, is insufficient inlow-temperature characteristics, high-rate discharge characteristics,and capacity maintenance rate.

Batteries D1 to F1 of comparative examples 2 to 4, in which theparticulate modified styrene-butadiene rubber is not used in thenegative electrode material mixture, are insufficient in safety andlow-temperature characteristics. In particular, in battery F1, whichused conventionally used PVDF as the binder in the negative electrodematerial mixture, the electrode plates were cracked during theproduction of the electrode group and have insufficient strengthalthough the amount of the binder was larger than that in the otherbatteries.

Table 2 indicates that the preferred amount of the particulate modifiedstyrene-butadiene rubber is 0.6 to 1.7 parts by weight per 100 parts byweight of the active material. Battery H1, which included the binder inthe amount of 0.5 parts by weight, had difficulty in production of anegative electrode and thus failed to be evaluated. Battery R1, whichincluded the binder in the amount of 1.8 parts by weight, wasinsufficient in high-rate discharge characteristics and capacitymaintenance rate.

Table 2 also indicates that the preferred amount of the thickening agentis 0.7 to 1.2 parts by weight per 100 parts by weight of the activematerial. Battery Q1, which included the thickening agent in the amountof 0.6 parts by weight, had difficulty in production of a negativeelectrode and thus failed to be evaluated. Battery K1, which includedthe thickening agent in the amount of 1.3 parts by weight, wasinsufficient in high-rate discharge characteristics and capacitymaintenance rate.

Table 2 further indicates that the preferred total amount of theparticulate modified styrene-butadiene rubber and the thickening agentis 1.3 to 2.4 parts by weight per 100 parts by weight of the activematerial. Batteries O1 and R1, of which the total amount was 2.5 partsby weight or more, were insufficient in high-rate dischargecharacteristics and capacity maintenance rate.

EXAMPLE 10

The positive electrode material mixture was prepared in the same manneras that employed in Example 2 except that 0.53 parts by weight of BM500Bas the binder and 0.27 parts by weight of BM700H as the thickening agentwere blended with 100 parts by weight of LiCoO₂. Using the resultantpositive electrode material mixture, battery T1 was produced andevaluated, as in battery C1 of Example 2. The results are shown in Table2.

Details of the binder and the thickening agent used in the positiveelectrode material mixture are as follows.

BM500B: Particulate modified acrylic rubber manufactured by Nippon ZeonCo., Ltd.

BM700H: A copolymer comprising an ethylene unit and a vinyl alcohol unitmanufactured by Nippon Zeon Co., Ltd.

In the absorption spectrum obtained by FT-IR measurement of BM500B,which is shown in FIG. 4, the intensity of the absorption peakattributed to the C═O stretching vibration in the 2-ethylhexylacrylateunit and the acrylic acid unit is about 10 times the intensity of theabsorption peak attributed to the C≡N stretching vibration in theacrylonitrile unit.

The measurement conditions, instrument, and the like are the same asthose for the measurement of the spectrum in FIG. 2. In FIG. 4, theabsorption peak observed near 2240 cm⁻¹ is attributed to the C≡Nstretching vibration in the acrylonitrile unit, and the absorption peakobserved near 1733 cm⁻¹ is attributed to the C═O stretching vibration inthe 2-ethylhexylacrylate unit and the acrylic acid unit.

The transmission spectrum obtained by FT-IR measurement of BM700H isshown in FIG. 5. The measurement conditions, instrument, and the likeare the same as those for the measurement of the spectrum in FIG. 2. InFIG. 5, the two absorption peaks observed near 2852 cm⁻¹ and near 2930cm⁻¹ are attributed to an OH group of the vinyl alcohol unit adjacent tothe ethylene unit.

As is found from the evaluation results of Table 2, battery T1 wassuperior in all the low-temperature characteristics, the high-ratedischarge characteristics, and the capacity maintenance rate to thebatteries of Examples 1 to 9, and was sufficient in safety. Thisindicates that the use of the particulate modified acrylic rubber as thebinder in the positive electrode material mixture dramatically improvesthe battery characteristics.

From the above results, it is evident that the present invention canprovide a non-aqueous electrolyte secondary battery that exhibits goodhigh-rate discharge characteristics and low-temperature characteristicsand ensures high safety.

EXAMPLES 11 TO 15 AND COMPARATIVE EXAMPLES 11 TO 18

Batteries A2 to M2 were produced in the manner described below, and thecharacteristics of the batteries were evaluated.

(i) Production of Negative Electrode

Negative electrode material mixtures for negative electrodes of therespective batteries were prepared using artificial graphites in theforms shown in Table 3 as the active material and binders shown in Table3. In Table 3, also shown are the specific surface area of theartificial graphite as the active material, the amount of the binder per100 parts by weight of the active material in the negative electrodematerial mixture, and the value of S_(Total)/W_(Binder) obtained bydividing the total surface area of the active material in the mixture bythe amount of the binder in the mixture. Note that CMC as the thickeningagent was mixed in the amount of 1.3 parts by weight per 100 parts byweight of the active material except for the case where PVDF was used asthe binder.

TABLE 3 Graphite Binder Surface Amount area (Part(s) byS_(Total)/W_(Binder) Example No. Battery Form (m²/g) Kind weight) (m²/g)Comparative A2 Bulk 4.5 BM400B 0.5 900 Ex. 11 Ex. 11 B2 Bulk 4.5 BM400B0.75 600 Ex. 12 C2 Bulk 4.5 BM400B 1 450 Ex. 13 D2 Bulk 4.5 BM400B 1.5300 Comparative E2 Bulk 4.5 BM400B 2 225 Ex. 12 Comparative F2 Bulk 4.5PVDF 4.5 100 Ex. 13 Comparative G2 Bulk 4.5 PVDF 7.5  60 Ex. 14Comparative H2 Bulk 4.5 SBR 1.5 300 Ex. 15 Comparative I2 Bulk 4.5 SBR 2225 Ex. 16 Comparative J2 Bulk 4.5 MPE 1.5 300 Ex. 17 Comparative K2Bulk 4.5 MPE 2 225 Ex. 18 Ex. 14 L2 Spherical 2.2 BM400B 0.7 314 BulkEx. 15 M2 Spherical 3.2 BM400B 1 320

The resultant negative electrode material mixture was applied to bothsurfaces of a core member made of 15 μm thick copper foil, rolled to athickness of 140 μm, and cut to a predetermined length, to obtain anegative electrode. A lead made of the same material as that of the coremember was connected to the negative electrode.

(ii) Production of Positive Electrode

Positive electrodes were produced in the same manner described above inrelation with the batteries A1 to S1.

(iii) Production of Battery

Batteries A2 to M2 were produced in the same manner as that for thebatteries A1 to S1, except for using a non-aqueous electrolytecontaining 1.0 mole/liter of LiPF₆ dissolved in the non-aqueous solvent.Each of the batteries was in the shape of a rectangle having a width of30 mm, a height of 48 mm, and a thickness of 5 mm, and had a nominalcapacity of 600 mAh.

During production of the batteries, whether or not the negativeelectrode material mixture came off from the core member was examined.In Table 4, mark “X” indicates that the mixture came off, and mark “◯”indicates the other results.

The resultant batteries were evaluated in the following points.

(i) Low-temperature Characteristics and Capacity Recovery Rate

Batteries A2 to M2 were charged until the battery voltage reached 4.2 Vat 600 mA and then discharged until the battery voltage decreased to 3 Vat 120 mA, in an atmosphere of 20° C. Subsequently, in an atmosphere of0° C., the batteries were charged until the battery voltage reached 4.2V at 600 mA and then discharged until the battery voltage decreased to 3V at 600 mA. Again, in an atmosphere of 20° C., the batteries werecharged until the battery voltage reached 4.2 V at 600 mA and thendischarged until the battery voltage decreased to 3 V at 120 mA.

The charged capacity (C_(LT)) at the charging until the battery voltagereached 4.2 V at 600 mA in an atmosphere of 0° C. was obtained for eachbattery. The results are shown in Table 4 as an indicator of thelow-temperature characteristics of the battery.

Also obtained were the capacity at the first-charging in an atmosphereof 20° C. and the capacity at the second charging in an atmosphere of20° C., and the ratio of the latter to the former was calculated. Theresults are shown in Table 4 as the capacity recovery rate expressed asa percentage.

(ii) Overcharge Test

The batteries used for the examination of the capacity recovery ratewere charged at 1260 mA at 20° C. until the battery surface temperaturebecame 80° C. The batteries were then left to stand for a while toexamine the change of the surface temperature. In Table 4, mark “X”indicates that the surface temperature rose to 90° C. or more, and mark“◯” indicates the other results.

(iii) Capacity Maintenance Rate

The capacity maintenance rates (C_(200th)/C_(1st)) of the batteries A2to M2 were obtained in the same manner described above with respect tothe batteries A1 to T1. The results are shown in Table 4.

TABLE 4 Capacity recovery Mixture C_(LT) rate OverchargeC_(200th)/C_(1st) Example No. Battery coming off (mAh) (%) test (%)Comparative A2 X — — — — Ex. 11 Ex. 11 B2 ◯ 156 99.3 ◯ 91.2 Ex. 12 C2 ◯110 98.8 ◯ 90.5 Ex. 13 D2 ◯ 65 86.1 ◯ 88.3 Comparative E2 ◯ 11 69.4 X75.4 Ex. 12 Comparative F2 X — — — — Ex. 13 Comparative G2 ◯ 47 84.4 ◯74.3 Ex. 14 Comparative H2 X — — — — Ex. 15 Comparative I2 ◯ 56 86.5 ◯74.5 Ex. 16 Comparative J2 X — — — — Ex. 17 Comparative K2 ◯ 39 81.1 ◯70.5 Ex. 18 Ex. 14 L2 ◯ 52 85.2 ◯ 86.7 Ex. 15 M2 ◯ 54 84.8 ◯ 87.7

From the results in Table 4, the followings are found.

The mixture came off from the core member when the surface area of thecarbon material per 1 gram of the particulate modified styrene-butadienerubber contained in the negative electrode was 900 m². This indicatesthat the strength of the negative electrode is weak when the surfacearea of the carbon material per 1 gram of the particulate modifiedstyrene-butadiene rubber in the negative electrode exceeds 600 m².

An abnormal temperature rise was observed in the overcharge test whenthe surface area of the carbon material per 1 gram of the particulatemodified styrene-butadiene rubber in the negative electrode was 225 m².This indicates that the safety of the battery may possibly be impairedwhen the surface area of the carbon material per 1 gram of theparticulate modified styrene-butadiene rubber in the negative electrodeis less than 300 m². This is presumably because absorption of Li by thenegative electrode active material becomes less easy and thus metallicLi is deposited on the surface of the active material. Also presumed isthat this phenomenon tends to easily occur during charging at a lowtemperature that causes large polarization.

From the viewpoint of the low-temperature characteristics, the surfacearea of the carbon material per 1 gram of the particulate modifiedstyrene-butadiene rubber in the negative electrode is most preferably inthe range of 450 to 600 m².

It is also found that the cycle life significantly decreases when abinder other than the particulate modified styrene-butadiene rubber isused in the negative electrode.

As described above, according to the present invention, it is possibleto secure the surface area of the active material of the negativeelectrode that can effectively contribute to the charging anddischarging reaction while securing the strength of the negativeelectrode. Therefore, using the negative electrode of the presentinvention, it is possible to obtain a non-aqueous electrolyte secondarybattery that is nicely balanced in the high-rate dischargecharacteristics, the low-temperature characteristics and the cycle life,and ensures high safety.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A non-aqueous electrolyte secondary battery comprising: a positiveelectrode comprising a compound oxide containing lithium; a negativeelectrode comprising a carbon material; as an active material and aparticulate modified styrene-butadiene rubber as a binder; a separatorinterposed between said positive electrode and said negative electrode;and a non-aqueous electrolyte; wherein the surface area of said carbonmaterial is 300 m² to 600 m² per 1 gram of said particulate modifiedstyrene-butadiene rubber wherein said particulate modifiedstyrene-butadiene rubber contains a copolymer comprising anacrylonitrile unit, a styrene unit, and a butadiene unit.
 2. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said copolymer is in a form of a core-shell type particle. 3.The non-aqueous electrolyte secondary battery in accordance with claim1, wherein, in a FT-IR absorption spectrum of said copolymer, theintensity of the absorption peak attributed to C≡N stretching vibrationin said acrylonitrile unit is 0.1 to 2 times the intensity of theabsorption peak attributed to C═C stretching vibration in said butadieneunit.
 4. A non-aqueous electrolyte secondary battery comprising: apositive electrode comprising a compound oxide containing lithium; anegative electrode comprising a carbon material; as an active materialand a particulate modified styrene-butadiene rubber as a binder; aseparator interposed between said positive electrode and said negativeelectrode; and a non-aqueous electrolyte; wherein said particulatemodified styrene-butadiene rubber contains a copolymer comprising anacrylonitrile unit, a styrene unit, and a butadiene unit, and whereinsaid copolymer is in a form of a core-shell type particle.
 5. Thenon-aqueous electrolyte secondary battery in accordance with claim 4,wherein, in a FT-IR absorption spectrum of said copolymer, the intensityof the absorption peak attributed to C≡N stretching vibration in saidacrylonitrile unit is 0.1 to 2 times the intensity of the absorptionpeak attributed to C═C stretching vibration in said butadiene unit.